Japanese Patent Application Publication No. 2001-239129 and Japanese Patent Application Publication No. 2006-308269 have disclosed provision of a heat recovery unit for recovery of heat from boiler exhaust gas upstream of desulfurization equipment or a dust collector in a boiler exhaust gas treatment line. Japanese Patent Application Publication No. 2001-239129 describes improvement of thermal efficiency of a boiler by heating boiler supply water with heat recovered with this heat recovery unit. Furthermore, Japanese Patent Application Publication No. 2006-308269 describes supply of water to a boiler after heating condensate in a steam turbine with heat recovered with the heat recovery unit and supply of air preheated with heat recovered with the heat recovery apparatus to a boiler.